Modern concrete structures are usually composites which utilize embedded materials such as steel and aggregate stone for strength and durability. Some examples of concrete structures include roadways and slab-on-grade foundations. The most common modern roadway paving materials are concrete and asphalt while slab-on-grade foundations are primarily concrete. Typical concrete exhibits extraordinary resistance to compressive forces but lower resistance to tensile forces and vibration. Concrete composites include steel bars, steel wire mesh or fiber materials added to vary the characteristics of the concrete and to increase its resistance to tensile forces and vibration.
The use of reinforced concrete composite was prevalent during construction of the interstate highway system in the United States. An updated form of steel reinforced concrete is still used today for urban traffic applications. Bridge decks and bridge footings also extensively employ reinforced concrete.
Roadway structures use other forms of surfacing that incorporate composite materials and steel reinforcement. Asphalt pavements are applied to a compacted gravel base generally at least as thick as the asphalt layer, but some asphalt pavements are built directly on the native subgrade. In areas with very soft or expansive subgrades such as clay, thick gravel bases or stabilization of the subgrade with cement or lime can be required. Subgrade stabilization can include use of steel mesh or reinforced concrete applied to the roadbed under asphalt pavement.
Slab-on-grade foundations are shallow foundations that are often constructed of concrete using reinforcing methods. Slab-on-grade foundations are typically prone to cracking due to defication when the subgrade becomes unstable. To reduce the effects of subgrade instability, steel, wire mesh, fiber composites and tension cables are employed in the concrete. Post-tensioning is a method of strengthening concrete using high-strength steel strands or cables, typically referred to as tendons. The tendons rest on anchors in the concrete. Tension is applied to the cables used to place the concrete in compression.
Two cases arise where it is often necessary to determine fluid leaks, subsurface fluid flow and subsurface anomalies such as land faults and buried inclusions.
In the first case, preparation of a roadbed or construction site for building requires that an analysis of subsurface anomalies and subsurface fluid flow in order to adequately plan for and construct either a road or a concrete foundation.
In this case, detecting subsurface anomalies is a difficult task because limited time and resources often prevents a sufficient sample size to be adequately confident that the site located is free from anomalies or subsurface fluid flow.
In the second case, a reinforced concrete or asphalt structure is already in place. Testing is necessary to determine subsurface leaks, subsurface anomalies or subsurface fluid flow after construction.
Detecting and locating fluid leaks and other subsurface anomalies beneath composite concrete and asphalt structures is often a difficult task because the concrete or asphalt prevents access to the soil underneath. Removal of sections of the concrete to inspect the subjacent soil is often required. Methods of the prior art often require destruction of large sections of the composite structures to locate leaks and other subsurface anomalies because the exact location of the leak or anomaly is unknown.
Methods currently exist for detecting and locating leaks from landfills, hazardous waste dumps, impoundments, and other outdoor fluid containment areas by measuring changes in the conductivity and/or resistivity of the adjacent soil. Daily et al. '406 discloses a “mise-a-la-masse” technique and an electrical resistance tomography technique.
The mise-a-la-masse technique involves placement of several electrodes, one inside and several outside the facility. An electronic potential is applied to different pairs of electrodes, but always includes the electrodes in the fluid containment facility. Voltage differences are then measured between various combinations of electrodes. The leak location is determined from the coordinates of a current source profile that best fits the measured potentials within the constraints of the known or assumed resistivity distribution. However, because the potentially leaking fluid must be driven to a potential, mise-a-la-masse methods can monitor for leaks in continuous fluid systems only, such as ponds, lined fluid containment areas, and tanks.
Electrical resistivity tomography (ERT) involves placing electrodes around the periphery of, beneath, or, in the case of subsurface containment vessels, above the facility. A known current is applied to alternating pairs of electrodes, and then the electrical potential is measured across other alternating pairs of electrodes. The measurements allow calibration of electrical resistivity (or conductance) over a plurality of points in the soil. Differences in resistivity correlate directly with migration of leaking fluid. However, Daily does not disclose a method or apparatus that allows the electrodes to be placed directly under the leak source, after construction of a structure or paved roadway.
Henderson '202 and '045 both disclose directly monitoring the soil subjacent to a fluid containment area by burying electrodes directly beneath the containment. Both Henderson patents disclose a plurality of four-plate electrode systems. A voltage and a known current are applied across the outer pair of plates. The resulting potential difference is measured across the inner pair. Henderson '045 also discloses a system of individual electrodes that, by varying the spacing between the electrodes impressing a current into the ground and the spacing of the potential measurement electrodes, can indirectly measure the resistivity at a calculated depth. However, Henderson '045 does not disclose a method of directly monitoring the subgrade beneath a structure without permanently burying the electrodes or a method to place electrodes beneath an existing structure.
Woods et al. '244 discloses a leak detection system for radioactive waste storage tanks. The system comprises a metal tank, an AC generator connected between the tank and a reference electrode, and a plurality of reference electrodes. When the generator is energized, it creates an electric field in the ground between the tank and the reference electrode. A voltmeter measures the potential difference between the sensing electrodes and the tank. A significant change in the potential at one or more of the sensing electrodes indicates that the tank has developed a leak. Woods et al. has a number of disadvantages. First, it requires an electrically conductive fluid container. Second, it requires that the electrodes be permanently buried in the soil surrounding the tank. Finally, it requires the use of an AC generator, which is less convenient than a DC power source.
Bryant '625 discloses a method and apparatus for creating an electrical resistivity map of the volume beneath a slab foundation by placing electrodes through a foundation, and applying a current through them. Bryant '625 further discloses a method for converting the measured potential to a resistivity value, assigning the resistivity value to a spatial coordinate, and storing a plurality of values in computer files. The apparatus includes an array of electrodes that are used to impress a known current in the soil and measure the resulting electrical potential of electrodes. Typically, a pair of electrodes is used to impress a constant current, and another pair is used to measure a voltage potential.
The array of electrodes is interconnected by electrical conducting cables that connect to the various electrodes at predetermined intervals. The interconnecting cables transmit electrical current that passes through certain electrodes to create the electrical field within the underlying and subjacent soil, and return electrical signals from other measuring electrodes that detect the electrical field within the soil. However, Bryant does not disclose the ability to switch current between nodes or to conduct an orderly permutation of voltage measurements between nodes.
None of the prior art is entirely satisfactory to locate fluid leaks beneath composite and reinforced concrete structures or to analyze them in near real time. For instance, it is not practical to electrify the potentially leaking fluid and because there may exist multiple sources of fluid, mise-a-la-masse is not a practical option. Nor is it practical to embed permanently a series of electrodes beneath an existing massive concrete structure or roadway to monitor soil resistivity. Further, because some of the ERT methods use multiple-plate electrodes where a large hole is bored to insert the electrodes into the subjacent soil making the method impractical and destructive. In addition, placing the electrodes around the periphery of a roadbed or foundation is less accurate compared to placing the electrodes directly beneath or adjacent the potential leak source.
The current state of the art is unsatisfactory because it does not provide a method to remotely change injection current locations or to conduct an orderly progression through a permutation of voltages and currents between nodes. Moreover, the state of the art does not provide for dynamically addressable sensors whose location and address can be changed on the fly.
Furthermore, the present state of the art requires that the electrodes be placed in a linear, regularly spaced grid pattern that does not provide needed flexibility in the physical layout of arrays of electrodes in multiple, non-linear arrangements. The present state of the art presumes the locations of the electrodes to one another. It may be necessary to arrange the electrodes in a non-linear grid if the physical layout of the concrete or underlying area to be measured prevents the layout of electrodes in a typical, linear grid arrangement. It may be necessary to locate certain electrodes in a non-linear pattern to accommodate obstructions in the existing structures foundation or to conform the layout of the grid to a particular stretch or curvature of a paved roadway or bridge, or other geographic anomaly. Furthermore, the prior art does not provide for a means to easily adjust an array of electrodes to avoid encapsulated steel structures such as rebar. Where a need arises to arrange the electrodes in a non-linear grid, it may be necessary to identify the spatial relationship of the electrodes. Likewise, it may be necessary to adjust the location of certain electrodes to accommodate obstructions and thus, it may be necessary to identify the new location.
Furthermore, the present state of the art requires that electrodes be placed according to a measured or surveyed pattern at the physical location. The requirement of physically measuring and placing electrodes is hampered by structures which include buildings, medians, or other obstructions which makes placing the electrodes accurately difficult. The inaccurate placement of electrodes leads to errors in the mathematical calculations required to analyze the locations of the anomalies and therefore reduces the overall efficiency and accuracy of the system.
Moreover, the state of the art does not provide for interchangeability of sensors. This limitation requires extensive time in installation and replacement of defective sensors. The uniqueness of sensors required by the prior art creates a need for specific sensors uniquely identified by their order in a specific grid.
Further, a need exists for accurate measurement of voltage offsets to accurately calibrate voltage readings among sensors.